1. Field of the Invention
This invention relates to means for accomplishing the efficient burning of biomass fuel materials, and more particularly, an apparatus and a method which make use of a catalytic converter for conditioning recirculated combustion product gases to eliminate pollutants while maintaining high operating temperatures at low rates of fuel consumption.
2. Description of Related Prior Art
Heating apparatus which burn biomass fuel materials have been known heretofore to include means for promoting increased combustion efficiency. Such heating devices can be classified into two general categories, the first category including apparatus using exhaust gas recirculation techniques and the second category including apparatus equipped with some type of secondary combustion chamber.
References relating to heating apparatus included in the first category disclose heating systems in which combustion gases are recirculated back to the firebox or combustion chamber so as to increase the temperature thereof and facilitate a more complete combustion of the unburned hydrocarbon pollutants.
In particular, U.S. Pat. No. 825,747 of Moldenhauer, et al. discloses a heating stove embodying the general concept of recirculating exhaust gases back to the combustion chamber and beneath the grate supporting the combustible fuel material thereon. The heating stove includes a return flue having a valve to control the amount of exhaust or combustion product gases returned to the combustion chamber where they are drawn through the grate and more completely combusted by the burning fuel.
Under the operating principles of the system described by Moldenhauer, et al., the exhaust gases recirculated to the combustion chamber burn freely and contribute to the heating effect of the stove, thereby accelerating ignition and increasing the rate of consumption of fuel materials supporting the primary burning process.
U.S. Pat. No. 4,242,972 of Sicard discloses a furnace including an elaborate combustion system which utilizes the basic exhaust gas recirculation technique disclosed by Moldenhauer, et al. The combustion system described by Sicard features an exhaust gas recirculation passageway having a particulate material storage container with means to agitate the particulate waste material to prevent excessive accumulation thereof. The suspended particles are pumped by a blower through the exhaust passageway to the combustion chamber and are burned therein. A damper included in the recirculation passageway controls the amount of exhaust gas returned to the combustion chamber.
Reduction of the amount of emitted pollutants is accomplished by recirculating hot combustion gases and a quantity of fresh air thereby to operate the furnace at a higher temperature to obtain more complete combustion. As in Moldenhauer, the principle of operation is to promote turbulence of the gases within the combustion chamber and accelerate ignition of both the combustion product gases and the combustible fuel material.
Other heating apparatus and stoves disclosing combustion systems which recirculate exhaust gases to increase the temperature and the burn rate within the combustion chamber include U.S. Pat. No. 602,962 of Sears, et al.; U.S. Pat. No. 664,751 of Hollingsworth; U.S. Pat. No. 2,603,195 of Permann; and U.S. Pat. No. 3,933,145 of Reich.
The second category of references includes patents disclosing the use of a catalytic converter or secondary burning chamber to combust more completely exhaust gases produced in a primary burning chamber.
U.S. Pat. No. 2,845,882 of Bratton discloses generally the use of catalytic units to reduce the number of pollutants emitted in the exhaust from an incinerator.
U.S. Pat. No. 4,319,556 of Schwartz, et al. discloses a wood-burning stove having a catalytic converter in which combustion product gases emitted from a primary combustion chamber are more completely combusted. The component parts of the stove include a combustion chamber, a catalytic converter, a heat exchanger, and a flue, all of which are functionally arranged in series relation in an open loop configuration. After the combustion product gases have been more completely combusted in the catalytic converter, the exhaust gases pass upwardly through the heat exchanger and outwardly of the system through the flue.
U.S. Pat. No. 4,330,503 of Allaire, et al. discloses a particular embodiment of a wood-burning stove having a catalytic converter interposed between a first outlet port of a combustion chamber and the inlet to a heat exchanger chamber. The outlet of the heat exchanger is structurally connected to both a flue and the combustion chamber through a three-channel passageway having a first control damper located at the outlet of the heat exchanger and a second control damper located at a second outlet port of the combustion chamber. The heat exchanger is in operational communication only with the flue; therefore, there is no recirculation of combustion product gases from the heat exchanger to the combustion chamber.
The operation of the two control dampers is coordinated to provide two separate, mutually exclusive open loop paths for the exhaust produced in the combustion chamber to pass through the flue direct into the atmosphere.
In the primary operational mode of the stove, the first control damper located at the inlet to the heat exchanger is opened to permit the combustion product gases to pass from the combustion chamber through the catalytic converter and heat exchanger to the flue and direct into the atmosphere. The second control damper located at the second inlet port to the combustion chamber remains in the fully closed position, thereby terminating a possible feedback path for the exhaust gases.
In a second operational mode, the first control damper is in the fully closed position and the second control damper is in an open position to eliminate back pressure within the stove to allow the exhaust gases to bypass the catalytic converter and heat exchanger and be discharged direct through the flue into the atmosphere during, for example, a fuel-loading operation. Again, recirculation of combustion product gases is neither contemplated nor operationally provided for.
Apparatus disclosing the use of a secondary combustion chamber performing the same operational function as the catalytic converter to combust more completely the exhaust gases include U.S. Pat. No. 4,180,052 of Henderson and U.S. Pat. No. 4,292,933 of Meier, et al.
None of the references discussed hereinabove discloses a heating apparatus utilizing both exhaust gas recirculation and a catalytic converter completely and efficiently to burn biomass fuel materials while maintaining elevated temperatures at reduced rates of combustion within the primary combustion chamber. A primary object of this invention, therefore, is to accomplish this task by providing an apparatus and a method which use a catalytic converter in combination with an exhaust gas recirculation passageway to eliminate the pollution content of the combustion product gases and return the residual exhaust gases to the combustion chamber to control the rate of combustion therein.
Another important object of this invention is to provide a heating apparatus and method which regulate the rate of combustion and consumption of fuel material by varying the quantity of recirculated combustion product residual gases returned to the combustion chamber.
A further important object of this invention is to provide a heating apparatus and method to recirculate conditioned exhaust gases back to the combustion chamber without automatically accelerating the ignition and increasing the consumption of fuel.
Still another important object of this invention is to provide a heating apparatus and method which produce and sustain elevated temperatures sufficient to support trouble-free operation of a catalytic converter without a requirement for accelerated rates of combustion and increased fuel consumption.